phases: move root phase assignment to it's own function
This moves the initial root phase assignment to it's own function. Future
patches which make phase calculations lazy will use this function to pre-fill
certain phases which can be deduced from the roots.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..